1. Field of the Invention
The present invention relates to zoom lens systems. In particular, the present invention relates to a zoom lens system that is suitable as an image-forming optical system in an image pickup apparatus such as a video camera, a silver-salt photographic camera, and a digital still camera.
2. Description of the Related Art
Image pickup apparatuses such as video cameras and digital still cameras that contain solid-state image pickup elements are equipped with image-forming optical systems. As an image-forming optical system, a zoom lens system that has a high zoom ratio and can exhibit high optical performance over the entire zoom range is required.
A known example of a zoom lens system that can fulfill these requirements is a four-unit zoom lens system consisting of four lens units, which are a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power that are arranged in that order from the object side towards the image side. Such a four-unit zoom lens system is known from examples of related art disclosed in Japanese Patent Laid-Open Nos. 8-304700, 2000-121941, and 2003-295053 (counterpart: U.S. Pat. No. 6,751,029). Specifically, the four-unit zoom lens system of these examples is of a so-called rear-focusing type that is configured to perform focusing by moving the second lens unit for magnification variation while compensating for an image-plane variation occurring from this magnification variation using the fourth lens unit.
Since a rear-focusing type four-unit zoom lens system performs focusing by moving relatively compact and lightweight lens units, the system is characterized in that the lens units can be driven with a small amount of force and that the focusing operation can be performed quickly.
Other examples of a rear-focusing type four-unit zoom lens system are disclosed in, for example, Japanese Patent Laid-Open Nos. 5-060971 (counterpart: U.S. Pat. No. 5,638,216) and 2005-242014 (counterpart: U.S. Pat. No. 6,972,909). The rear-focusing type four-unit zoom lens system according to these examples has a high zoom ratio and can exhibit high optical performance over the entire zoom range.
Furthermore, a compact four-unit zoom lens system is known from Japanese Patent Laid-Open No. 11-023965 (counterpart: U.S. Pat. No. 6,084,722). In this four-unit zoom lens system, the second lens unit consists of three negative lens elements and a single positive lens element.
In Japanese Patent Laid-Open No. 11-023965, the second lens unit consisting of the three negative lens elements and the single positive lens element that are arranged in that order from the object side towards the image side allows for high optical performance over the entire zoom range.
In this case, the refractive powers of the third lens unit and the fourth lens unit can be increased so that the focal length of the system at the wide-angle end can be shortened without having to excessively increase the refractive power of the second lens unit. This can allow for better compensation for the aberrations at the wide-angle end, particularly for transverse chromatic aberration.
By selecting an appropriate lens material and refractive power for each of the lens elements constituting the second lens unit, variations in the curvature of field and transverse chromatic aberration can be properly compensated for over the entire zoom range from the wide-angle end to the telephoto end.
In order to achieve compactness, high zoom ratio, and high optical performance over the entire zoom range in a zoom lens system, it is necessary to set appropriate moving conditions for the lens units for zooming, appropriate refractive powers for the lens units, and appropriate lens configurations for the lens units.
In particular, in a four-unit zoom lens system of a rear-focusing type described above, setting an appropriate lens configuration for the second lens unit used for magnification variation and appropriate refractive powers for the third and fourth lens units are significant factors for achieving a high zoom ratio and high optical performance.
If these configurations are not set properly, it becomes difficult to achieve a zoom lens system that has high optical performance over the entire zoom range from the wide-angle end to the telephoto end.